1. Field of the invention
This invention relates to a silicon carbide light emitting diode having a pn junction, and more particularly, it relates to a silicon carbide light emitting diode having a pn junction, which can emit visible light of a shorter wavelength corresponding to a color in the range of from green to purple.
2. Description of the prior art
Since light emitting diodes are a small luminescent source which dissipate a significantly little amount of power, and can provide a stable light emission of high brightness, they are widely used as a display element in a variety of display units. They are also used as a light source for reading information records in a variety of information processing units. Although light emitting diodes capable of emitting light of a color in the range of from red to green have been put to practical use, light emitting diodes capable of emitting visible light of a shorter wavelength corresponding to a color in the range of from blue to purple have not yet attained a performance sufficient for practical use.
The color of light emitted from a light emitting diode depends on the semiconductor material used therefor. Semiconductor materials which can be used for blue light emitting diodes are limited to silicon carbide (SiC) which is a IV-IV group compound semiconductor, gallium nitride (GaN) which is a III-V group compound semiconductor, and zinc sulphide (ZnS) or zinc selenide (ZnSe) which are II-VI group compound semiconductors. With these semiconductor materials, extensive research and development for blue light emitting diodes has been made, but mass production of blue light emitting diodes with a brightness and stability sufficient for practical use has not yet been realized.
For the structure of light emitting diodes, a pn junction structure is most suited because electrons and holes as carriers can be injected in a light emitting region with high efficiency. However, among the above-mentioned semiconductor materials for blue light emitting diodes, each of the GaN, ZnS, and ZnSe semiconductors cannot be used for pn junction light emitting diodes since it is difficult to obtain p-type crystals, or, even if these crystals are obtained, they have high resistance and are very unstable. Therefore, a metal-insulating layer-semiconductor (MIS) structure using a thin insulating layer has been employed instead of a pn junction structure. Light emitting diodes with such an MIS structure have the disadvantages of having uneven device characteristics and of providing unstable light emission.
On the other hand, silicon carbide can be used for pn junction light emitting diodes since both p-type crystals and n-type crystals can readily be obtained. Moreover, they have the advantage that a growth method suitable for mass production, such as a liquid epitaxial growth (LPE) method, chemical vapor deposition (CVD) method, or the like, can be used in producing such a pn junction light emitting diode. Many reports have already been made on silicon carbide blue light emitting diodes with aluminum (Al) used as a luminescent center (see, e.g., M. Ikeda, T. Hayakawa, S. Yamagiwa, H. Matsunami, and T. Tanaka, Journal of Applied Physics, Vol. 50, No. 12, pp. 8215-8225, 1979; L. Hoffmann, G. Ziegler, D. Theis, and C. Weyrich, ibid., Vol. 53, No. 10, pp. 6962-6967, 1982).
FIG. 3 shows a conventional silicon carbide light emitting diode having a pn junction. The silicon carbide light emitting diode has a structure in which a n-type SiC single-crystal thin layer 7 and a p-type SiC single-crystal thin layer 8, both of which constitute the pn junction, are successively formed on an n-type SiC single-crystal substrate 1. Moreover, an ohmic electrode 9 for n-type SiC and an ohmic electrode 10 for p-type SiC are formed on the n-type SiC single-crystal substrate 1 and the p-type SiC single-crystal thin layer 8, respectively. In this light emitting diode, nitrogen (N) donors are used as dopants for generating carriers in the n-type SiC single-crystal thin layer 7, and aluminum (Al) acceptors are used as dopants for generating carriers in the p-type SiC single-crystal thin layer 8. As the dopants for generating carriers, gallium (Ga) acceptors or boron (B) acceptors may also be used. The n-type SiC single-crystal thin layer 7 contains Al as a luminescent center for the blue color.
Such a pn junction light emitting diode further contains an appropriate amount of Al acceptors in the n-type SiC single-crystal layer 7, and utilizes light emission by the radiative recombination between electrons trapped by the N donors and holes trapped by the Al acceptors, or by the radiative recombination between electrons in the conduction band and holes trapped by the Al acceptors, in the region of the n-type layer in the vicinity of the pn junction. That is, the Al acceptors are used both as the dopants for generating hole carriers and as the luminescent center. For this reason, it is not possible to control either a carrier concentration or a luminescent process in the p-type layer and the n-type layer independently of each other, thereby making it difficult to improve luminous efficiency. Moreover, electrons or holes cannot be sufficiently trapped by the N donors and the Al acceptors since the energy levels of the N donors and the Al acceptors are not fully away from the conduction band and the valence band, respectively. Therefore, the conventional silicon carbide light emitting diode has the disadvantage in that high luminous efficiency cannot be obtained.